-LRB- CNN -RRB- It started with a sore throat on Thanksgiving and an antibiotic from a friend who wanted to help .

Now 19-year-old Yaasmeen Castanada is fighting for her life inside a California hospital 's burn unit , suffering from an allergic reaction that 's so severe she has large open wounds all over her body .

`` It is heartbreaking , every day is a different look . Every day , she 's like , shedding away . ... Overnight , it 's a whole different person that you 're looking at , '' Martha Hughes , Castanada 's aunt , told CNN affiliate KABC .

Doctors diagnosed Castanada with Stevens-Johnson Syndrome , a rare disease that can be triggered by antibiotics or other medications .

`` When she took the medication , she started having a hard time breathing , and she told her mom that her lips were burning , her throat , her eyes , they got so red that she could n't talk . So she rushed her to the ER , and that 's when they diagnosed her with the disease . And from there it has just spiraled to a nightmare , '' Hughes said .

Now Castanada , the mother of a 4-month-old , is in critical condition at the University of California , Irvine , burn center .

Her prognosis is good , even though the disease has a high mortality rate , according to Dr. Victor Joe , the center 's director .

But the situation , Castanada 's family says , has been devastating .

`` Just unreal , just watching your daughter burn in front of you , literally , burn in front of you , '' her mother , Laura Corona , told KABC . `` Every day , a new blister , a new burn , a new scar . And she 's just , ` Mommy , I want to go home . ' And I ca n't take her home . I ca n't put water on her lips . ''

Mom : ` Do n't share medication '

On a website created to raise funds for Castanada 's care , her mother said the harrowing ordeal began soon after her daughter took the medicine .

`` A friend offered her an antibiotic pill that she had from a previous illness , '' Corona wrote . `` She was thinking that it would help her . This would be the biggest mistake of her life . ''

Now , Corona says she 's hoping to spread the word so others do n't make the same mistake .

`` Do n't share medication . Do n't give someone else your medication . Do n't offer medication , '' she said .

She also advises parents to find out what their children are allergic to -- before it 's too late .

Doctor : Reaction causing skin to separate

At first , doctors diagnosed Castanada with Stevens-Johnson Syndrome , which refers to a condition where between 10 % and 30 % of the skin on the body is affected , Joe said . Now she 's experiencing Toxic Epidermal Necrolysis , the diagnosis when more than 30 % of the body is affected . Joe estimates that 65 % of Castaneda 's skin and mucus membranes have been affected .

The allergic reaction is causing layers of Castaneda 's skin to separate , Joe said , creating lesions that grow into large open wounds .

`` Patients can experience problems with taste , swallowing , eyesight and sexual functions can be affected . In Yaasmeen 's case , we are particularly concerned because her eyes have been affected . This can cause scarring of the corneas , which could lead to permanent blindness , '' he said . `` We are trying to prevent that from happening . ''

Photos on the fundraising website show Castanada lying in a hospital bed , with openings for her eyes cut from the bandages that cover her .

As part of her treatment for the disease , doctors have wrapped her body in a special dressing , Joe said .

`` We have chosen to place a dressing that adheres to the open wound , which allows her skin to heal without having to remove the bandages to wash the wounds , '' he said .

Mortality for those suffering from Stevens-Johnson Syndrome and Toxic Epidermal Necrolysis can be as high as 25 % in adults , but tends to be lower with early treatment , according to the Merck Manual .

Though it 's uncommon , Joe said his hospital has treated around six cases in the past year , because the burn center has experience treating open wounds .

`` This is very sobering . The fact that you can get a life-threatening situation from taking a medication . It can happen , and most people do n't think twice about taking pills for things , '' Joe said . `` In fact , most of the time you do have some sort of side reaction to medication , just not this severe . ''

After recovering from Stevens-Johnson Syndrome , patients usually only have minor issues with their skin , such as dryness , Joe said .

`` Hopefully new skin will come in , '' Corona told KABC . `` I 'm just there watching . All I can tell her is , `` Hang on , hang on . It 's almost over . ' ''

CNN 's Joseph Netto and Deborah Goldschmidt contributed to this report .

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An allergic reaction to an antibiotic sent Yaasmeen Castanada to a hospital burn unit

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`` It has just spiraled into a nightmare , '' her aunt told CNN affiliate KABC

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A doctor says the teen 's prognosis is good , but she 's in critical condition